Video images are commonly transcoded to a bitstream form by an encoder. The bitstream is stored according to an encoding type that satisfies the conditions of the encoder.
MPEG requires syntax and semantics for the conditions of a bitstream.
Syntax refers to the structure or form and length of data, and indicates the order of expressing the data. In other words, syntax is for conforming to the grammar for encoding/decoding operations and defines the order, length and form of the data included in the bitstream.
Semantics indicates what each of the bits constituting the data means. That is, semantics shows the meaning of each element in the bitstream.
Therefore, various types of bitstreams can be generated according to the encoding conditions or applied standard (or codec) of the encoder. Generally, each standard (e.g., MPEG-1, MPEG-2, MPEG-4, MPEG-4 AVC, etc.) has different bitstream syntax.
Therefore, every bitstream that is encoded according to a different standard or encoding condition has different forms of syntax, semantics and decoding process, and it is required that a decoder corresponding to the encoder be used in order to decode the bitstream.
As described above, there has been a restriction in the conventional bitstream decoder to satisfy the conditions of the encoder, and this restriction has made it difficult to realize a unified decoder that could address multiple standards.